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June 24, 2014 -- Scientists say they may be able to explain how ongoing stress raises the risk of having a heart attack.

They say stress triggers our bodies to make a surplus of disease-fighting white blood cells. That in turn can boost inflammation in the arteries of people with a condition called atherosclerosis, where the artery walls are thickened by a buildup of plaque.

Inconclusive

Medical research has so far been inconclusive about how stress raises the risk of heart disease. For instance, stress itself might be responsible, or it could be that high levels of stress contribute to other risk factors like high cholesterol or high blood pressure.

Studies have also linked stress to changes in the way blood clots, which raises the risk of a heart attack.

Scientists from Massachusetts General Hospital and Harvard Medical School looked at how stress affected the white blood cell counts of 29 medical workers in a hospital's intensive care unit after 1 week. This workplace was chosen because of its fast-paced nature, where staff regularly have to make life-or-death decisions.

The authors of the study, published in Nature Medicine, report that when they compared blood samples taken from the volunteers during work to samples taken when off-duty, the white blood cell count was higher in the work samples.

Mice Studies

Next, the scientists exposed mice to stressful conditions by isolating them or tilting their cages until their bodies made excess white blood cells. These mice were already prone to hardening and narrowing of the arteries.

The scientists found that this seemed to trigger plaque inflammation in the arteries of the mice. They say these plaques resembled the way in which excess white blood cells clump together on the inner wall of arteries in humans, where they block circulation or are prone to break off and travel to other parts of the body, potentially triggering a heart attack.